1. Field of the Invention
The invention relates to a wavelength tunable semiconductor laser including a laser active region, a tunable frequency filter and a photon emission region from which photons are injected into the laser active region.
2. Background Information
The periodical, Electronics Letters, Volume 24, No. 8, of Apr. 14, 1988, discloses such a semiconductor laser at pages 503-505. This laser is composed of three longitudinally integrated regions, that is regions that are arranged next to one another in the longitudinal direction: a laser active region for light amplification, a controllable phase shifting region and a distributed Bragg reflector constituting the tunable frequency filter. It is disposed on the side of the laser active region and determines the Bragg wavelength of the waveguide region. Such a semiconductor laser is also called a multi-segment laser. In order to tune the wavelength of the laser light, currents are injected into the phase shifting region and into the waveguide region in the area of the Bragg grating; these currents change the Bragg wavelength because the refractive index in this region changes due to the plasma effect and the band filling effect.
The publication covering the InP-Workshop at the Heinrich Hertz Institute, Berlin, May 8-9, 1989, discloses a "continuously tunable laser triode"; this semiconductor laser is transversely integrated.
Only a single control current is required to tune its effective refractive index. In order to reduce the absorption of photons from the region of the Bragg grating, this region is produced of a material which has a higher band edge than the laser active region. However, this reduces the effect of the charge carrier injection in the laser active region since it is in the vicinity of the band edge of the semiconductor material that the plasma effect reaches its maximum. Varying the band masses of electrons and holes in the region of the Bragg grating also does not permit an injection of significantly more photons into the laser active region. It merely results in a suppression of the stimulated emission of photons in the region of the Bragg grating.